1. Field of the Invention: This invention relates to the gasification of coal and similar carbonaceous solids and is particularly concerned with a method for the removal of ash during fluidized bed coal gasification and related operations.
2. Description of the Prior Art: A number of different processes for the gasification of coal and similar carbonaceous solids have been developed in recent years. Among the most promising of these are fluidized bed processes in which feed coal particles are devolatilized to produce hydrocarbon gases and char and char is reacted with steam to form synthesis gas. The reactions involved, which may be carried out in a single vessel or in two or more reactors, are highly endothermic and require that large amounts of heat be supplied. This is generally done by burning a portion of the char, either by injecting oxygen into the fluidized bed with the steam or by withdrawing char from the bed, passing it to a separate combustion zone, and then returning hot char particles to the fluidized bed reaction vessel. The gasification and combustion reactions which thus take place result in the production of significant quantities of ash. The ash not carried overhead with the product gases tends to accumulate in the system and must be removed if the process is to operate continuously.
Several different methods for coping with the ash removal problem have been proposed. Much of the early coal gasification work was carried out with slagging type gasifiers which were operated at temperatures above the ash fusion point and therefore resulted in the formation of an ash which could be quenched and withdrawn as slag from the lower part of the gasifier. Such a system is useful for the removal of ash from gasifiers designed for the production of synthesis gases of low methane content but poses problems where high B.t.u. product gases are desired. The high temperatures required to melt the ash tend to crack any methane present and hence the B.t.u. content of the product gas will be low. An alternate procedure is to withdraw a portion of the char from the system continuously. This, of course, has disadvantages in that it results in the discharge of substantial quantities of carbon that could otherwise be employed in the gasification process.
During coal gasification, the density of the char particles increases as carbon is consumed. It has been suggested that this density difference be used to permit the separation of char particles having high ash contents from those which contain greater quantities of carbon and less ash. To accomplish this, it has been proposed that a portion of the steam or other reactant gas to be used in the fluidized bed be injected into the lower portion of the fluidized bed reaction vessel at a relatively low rate which is sufficient to suspend the lighter particles of low ash content but insufficient to suspend the heavier particles. The additional gas required to maintain the bed in the fluidized state is introduced at a somewhat higher rate above the lower gas inlet. Coal particles fed into such a system near the top of the reaction vessel become fluidized and circulate within the bed. Lighter particles of low ash content which find their way into the zone below the level at which the main fluidizing gas is injected are entrained and carried back into the bed. Heavier particles of higher ash content which fall into the zone below the main gas injection level and cannot be entrained tend to accumulate in the lower portion of the vessel and can be withdrawn as a high ash content stream.
A system of the type described above has advantages over earlier methods proposed for the removal of ash but requires very careful control of the gas velocities if effective separation of the particles is to be obtained. Because of the erratic movement of particles within the fluidized bed, many of the heavier particles tend to remain suspended in the upper part of the system and may never reach the lower zone below the main gas inlet. This is particularly true in systems where particles are continuously circulated between the fluidized bed and a separate combustion vessel. As a result, the amount of ash removed from such a system by the above method may tend to be low and the amount of carbon withdrawn with the ash which is removed may tend to be relatively high. This reduces the efficiency of the system, may eventually result in deposit problems despite the removal of a pair of the ash, and may be accompanied by other difficulties. Moreover, the internal equipment which must be provided in the lower part of the fluidized bed gasifier if the above method is to be used may interfere with the return of hot char to the lower end of the gasifier, thus rendering the entire system inoperative. Efforts to avoid these and related problems have in the past been largely unsuccessful.